Diagnose a drop to zero in Facebook event creation

The number of events being created on Facebook has fallen by 100%. How would you work out why? ```hint What can reach zero Before listing causes, ask what kind of cause could take a metric for a product this large all the way to zero rather than just lower. ``` ### Clarifying Questions - Where is the drop showing up: in a dashboard or report, or are people also telling us they cannot create events? - Compared with what period, and since when: yesterday, the same day last week, or a longer trend? - Is it zero everywhere, or 100% down only on one platform, in one country or from one entry point such as Groups or Pages? - Are related numbers, such as views of existing events, invitations and RSVPs, also affected? ### What a Strong Answer Covers - What the size of the drop implies about which causes are plausible - Checking whether the number itself is right before diagnosing the product - A segmentation plan by platform, app version, region and creation entry point to locate the failure - Lining up internal changes with the start of the drop and checking them in a sensible order - The response and prevention: escalation, rollback, telling the people who rely on the metric, and earlier detection ### Follow-up Questions - Suppose the drop is 100% on Android and normal everywhere else. What do you check next? - If the data pipeline is healthy and you can create an event yourself without trouble, what else could explain the zero? - How would your approach change if the drop were 10% instead of 100%?